1. Field of the Invention
The present invention relates to a semiconductor device having a circuit constituted by a thin film transistor (hereafter referred to as a TFT), and to a method of manufacturing the semiconductor device. For example, the present invention relates to an electro-optical device, typically a liquid crystal display device, and to electronic equipment loaded with the electro-optical device as a part. Note that, throughout this specification, the term semiconductor device, indicates general devices which function by utilizing semiconductor characteristics, and that the above electro-optical device and electronic equipment fall under the semiconductor device category.
2. Description of the Related Art
Techniques for crystallizing and increasing crystallinity of an amorphous semiconductor film formed on an insulating substrate such as glass by performing heat treatment, laser annealing, or both heat treatment and laser annealing have been widely researched. Silicon is often used in the semiconductor film.
Crystalline semiconductor films obtained in accordance with the above techniques are made from a plurality of crystal grains, and therefore it is referred to as polycrystalline semiconductor films. The polycrystalline semiconductor films have extremely high mobility in comparison with amorphous semiconductor films. A monolithic type liquid crystal electro-optical device (a semiconductor device in which thin film transistors (TFTs) for driving pixels and for a driver circuit are manufactured on one substrate), which cannot be realized by a semiconductor device manufactured using a conventional amorphous semiconductor film, for example, can therefore be manufactured if a polycrystalline semiconductor film is utilized.
The polycrystalline semiconductor films are thus semiconductor films which have extremely good characteristics compared to amorphous semiconductor films. This is why the above stated research is being carried out. For example, it is necessary to perform a heat treatment at temperature equal to or greater than 600° C., and for a heat treatment time equal to or greater than 10 hours, preferably equal to or greater than 20 hours, to perform crystallization of an amorphous semiconductor film. Substrates, which can withstand these crystallization conditions, include quartz substrates, for example. However, quartz substrates are high cost, poor for processability, and in particular, they are extremely difficult to be processed into a large surface area. Increasing the surface area of the substrate is indispensable for raising mass production efficiency, in particular. Work towards increasing the surface area of the substrate for increasing mass production efficiency has been remarkable in recent years, and a substrate size of 600×720 mm is becoming a standard more and more for newly constructed mass production lines.
The processing of a quartz substrate into this type of large surface area substrate is difficult with present techniques, and even if it were possible, the price thereof will not be reduced to the extent that the industry may be established. Glass is available, for example, as a material which can easily be manufactured into a large surface area substrate. A glass substrate referred to as Corning 7059 exists, for example, as this type of glass substrate. Corning 7059 is extremely low cost, has good processability, and is easily made into a large surface area substrate. However, Corning 7059 has a softening temperature of 593° C,. and has a problem in heat treatment at 600° C. or higher.
Corning 1737 exists as one of glass substrates with a relatively high softening temperature. The softening temperature is high at 667° C. If an amorphous semiconductor film is formed on a Corning 1737 substrate, and the substrate is then placed in a 600° C. atmosphere for 20 hours, there is almost no change in shape of the substrate which will influence manufacturing. However, a heat treatment time of 20 hours is too long in a mass production process, and from the viewpoint of costs, it is preferable to lower the heat treatment temperature of 600° C., even by a small amount.
A novel method of crystallization has been proposed in order to resolve such problems. The method is disclosed in detail in Japanese Patent Application Laid-open No. Hei 7-183540. A simple explanation thereof is presented here. First, a very small amount of an element such as nickel, palladium, or lead is added to an amorphous semiconductor film. Methods such as plasma processing, evaporation, ion injection, sputtering, or liquid application can be utilized as the method for the addition. After the addition, if the amorphous semiconductor film is placed, for example, in a 550° C. nitrogen atmosphere for 4 hours, a polycrystalline semiconductor film having good characteristics can be obtained. The optimal heat treatment temperature and heat treatment time for crystallization are dependent upon the amount of the element added and the state of the amorphous semiconductor film.
A method of crystallization of an amorphous semiconductor film by heat treatment is described above. On the other hand, the temperature of the substrate does not increase very much with crystallization by laser annealing and high energy can be imparted to only the amorphous semiconductor film, and therefore substrates such as plastic substrates can also be used, in addition to glass substrates with low softening temperature.
Lasers such as an excimer laser, and an Ar laser can be given as examples of lasers which can be used in laser annealing. A method for performing laser annealing in which: a pulse laser beam of a high output is processed into a square spot of several centimeters in size, or into a linear shape having a length equal to or greater than 10 cm, by an optical system on an irradiation surface; and in which the laser beam is then scanned (or the laser beam irradiation position is moved relatively with respect to the irradiation surface), has high productivity and is industrially superior. This method is therefore used preferably.
In particular, when a laser beam of which the shape is linear on the irradiation surface (hereafter referred to as a linear beam) is used, the entire surface to be irradiated can be irradiated by scanning the linear beam in only a perpendicular direction to the linear direction of the linear beam, while it is necessary to scan forward and backward, and left and right in the case of using a spot laser beam, and productivity is therefore high. Scanning in a perpendicular direction to the linear direction is performed because it is the most efficient scanning direction. It is becoming a present tendency to use pulse emission excimer laser processed into a linear beam by a suitable optical system for laser annealing, due to this high productivity.
Further, there is also a method of performing crystallization of an amorphous semiconductor film by laser annealing after crystallization is performed by heat treatment. It may make the characteristics of the semiconductor film better to perform this method, compared to performing only heat treatment or only laser annealing. In order to obtain high characteristics, the conditions of heating and laser annealing require to be optimized. When For example, the thin film transistor (TFT) is manufactured by a known method, using a polycrystalline semiconductor film that is obtained in accordance with the method described above, electric characteristics of the TFT may be markedly improved.